Just looking at some previous times Barcelona have qualified early to check out what their team on the final group day was:
2017-18: Cillesen; Semedo, Pique, Vermaelen, Digne; Vidal, Rakitic, D.Suarez, Gomes; L. Suarez, Alcacer.
Beat Sporting 2-0 at home.
2016-17: Cillesen; Vidal, Umtiti, Mascherano, Digne; D. Suarez, Iniesta, Gomes; Turan, Messi, Alcacer.
Beat Gladbach 4-0 at home.
2015-16: Ter Stegen; Adriano, Vermaelen, Bartra Alba; Rakitic, Samper, Kaptoum; Messi, Sandro, Munir.
Drew 1-1 with Leverkusen away.
I'd say they definitely weaken the team. Though at Barcelona, a weakened team is still ridiculously strong. If I had to hazard a guess at the potential team they'd put out against us, I'd say something like this:
Cillesen
Semedo - Lenglet - Umtiti - Alba
Rafinha - Vidal - Arthur
Dembele - Messi - Munir
Though it may even be stronger than that and whether they feel Busquets should play or not. I think the lack of Busquets would help us greatly. I think Messi will definitely play to get his match fitness back up following his injury. Even if it is a similar team to that, it's still ridiculously strong and we'd have to be at our best for 90 minutes to win it, let alone draw.